So here is what I believe.

Anytime you use a system which leaks information you should be expecting that someone is listening to it. So be wise and use encryption. So if you are wearing a t-shirt that says "I am cheating on my girl/boy-friend" you cant expect to stop people from reading (and thus spreading) it. If it is super important for the message to be private it is your responsibility make sure it does not get out, dont wear the t-shirt/encrypt it. Also, remember that encryption is only fool proof so spend as much money into security as the information is worth.

On the other hand interacting with this system is a bit more sketchy subject. It would be quite wrong to try to stop a message from making it to its destination. Unless ofcurse, the message is going through "your medium" or is aimed to circumvent your security. For example I block a lot of light from exiting my house, if you shine a flashlight into my house for what ever reason expect it to be hindered.

Using a medium without explicit permission in a manner which does not disrupt a system? If there is a mechanism which is reasonable and is inteded to keep you out it is probably safe to say, you should not be using it. Ie: A mall is generally private property but they do not make an attempt to keep everyone out (unless they are closed and few other exceptions) so in that case you ought to be able to use their heat/light/shelter when you walk in. On the other hand if a house is locked it'd be wrong to go as far as you can and start playing around with a lock.

And finally a system might say explicitly that you are not welcome but not much of an attempt is made from stopping you. In this case you should not embark on it. Ie: "Posted, Private property, no tresspassing" on private land such as a forest. If this type of system is yours make sure to mark it clearly.

In your case I'd say the mall scenario applies, party did not clearly label that they dont want you there nor did they invest in ample security to keep you out. You should be able to use it within reason (nothing illigal)